Understand the Unfairness of BitTorrent

BitTorrent (BT) is the most popular P2P file-sharing appli- cation. Its tit-for-tat mechanism aims to guarantee the effi- ciency and fairness of sharing. However, while BT's down- load efficiency has been proven, we find that the current pro- tocol suffers seriously from unfairness, in the sense that cer- tain peers will always serve as Super Peers. In this paper, we report on experiments conducted to pinpoint the cause of unfairness. The results indicate that the occurrence of Super Peer has a strong correlation with the bandwidth between the initial seed and the peer, and a weak correlation with the start time of the peer.